Question

# A ball is thrown up from the top of a tower at 20 m/s velocity. It took the ball 6 sec to fall to the bottom. How high is the tower? For how long was the ball above the tower?

A

2 sec

B

4 sec

C

6 sec

D

1 sec

Solution

## The correct option is B 4 sec When the ball was going up till B then its displacement was increasing. When the ball started to fall from B to C its displacement was still positive but decreasing when ball is at C its displacement from the starting position is 0. So when the ball comes back to C after falling from B then its displacement becomes O So, we know ball was thrown with 20 m/s up                                 u = + 20 m/s                                 s = 0 Acceleration         = - 10 m/s2 As always the bell is under the influence of gravity So a=−10m/s2 s=ut+12at2 o=20.t+12×(−10)t2 t(20 - 5t) = 0 t = 0 or 4 sec The equation gives 2 answers for t at which s = 0 By analyzing we see that at t = 0 particle was at starting point only so displacement = 0 At, t = 4 s = 0 Means ball comes back to starting point after its journey. Physics

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